Rolling element bearings are used in a variety of applications. Bearings are often preloaded, which requires the application of an axial force on the bearings. This preload allows the bearings to maintain acceptable stiffness and minimizes vibrations and noise of the bearings in the presence of an axial clearance or other slight misalignments caused by wear, thermal expansion, or tolerances. Proper levels of preload may increase bearing fatigue life, give predictable levels of system rigidity, control ball skidding under high acceleration, and reduce repetitive run-out. However, it is important to maintain the desired preload levels, as problems may occur when preload levels are too high or too low. When preload levels are too high, the bearings may experience problems such as a shortened life due to high level of fatigue on the bearings, increased noise, and increased torque levels. When preload levels are too low, the bearings may have fretting corrosion caused by vibrations of the bearing elements. Therefore, it is important to maintain acceptable levels of preload for each bearing throughout the entire range of operating conditions specified for each bearing.
In many of these applications, such as in the aviation and spacecraft industries, it is desirable that the system components be as lightweight as possible. Bearings are often constructed of different types of materials to satisfy the need for low-weight but durable assembly components. For instance, hybrid bearings having steel rings and ceramic rolling elements have been developed. Steel is used for the rings of the rolling element bearings, because the rings of rolling element bearings typically experience tensile stresses when interference fitted onto a shaft. The rolling elements of the bearings do not experience tensile stresses like the bearing rings, and may be constructed of a lighter-weight material that may have good performance characteristics in compression. One material which is used to construct these rolling elements is a ceramic material, which is lightweight, durable, and has low rolling friction.
Using two different types of materials for bearings elements is beneficial for increasing the life and reducing the overall weight of the system. However, because these rolling elements must perform at a wide range of operating temperatures, problems arise because the materials have different rates of thermal expansion. For instance, steel may have a coefficient of thermal expansion (CTE) of 5.6 μin/in*° F., while the ceramic rolling element may have a much lower CTE of approximately 1.6 μin/in*° F. The differences in the rates of thermal expansion cause changes to the preload when the bearings undergo changes in operating temperatures. For instance, bearings elements designed to operate in space may have operating temperature ranges of 180° F. to −65° F. For systems operating under large temperature fluctuations, problems may arise because of changing preloads due to the differing rates of thermal expansion of the bearing rings and the bearing rolling elements.
Methods of passively controlling preloads have been developed, such as those disclosed in U.S. Pat. No. 6,135,641. However, these methods still rely on analytical predictions to control bearing preloads.
Therefore, an improved method of controlling preload levels for hybrid rolling elements bearings is needed.